1. Field of the Invention
The present invention relates generally to a method and apparatus for performing upflow vertical tube evaporation, and more particularly it relates to an improved system for distributing a liquid feed among a plurality of vertical tubes.
Conventional vertical tube evaporators typically comprise a multiplicity or bundle of parallel heat transfer tubes extending through a heating vessel, such as a steam jacket. Such vertical tube evaporators may operate in either a downflow or an upflow mode, the latter being the subject of the present invention. In the upflow configuration, the liquid to be evaporated, referred to as the distilland, enters the evaporator through a plenum formed below the open heat transfer tubes. Heat is supplied to the liquid by the steam condensing on the outer surfaces of the tubes, and an upward flow of the distilland is induced by the flow of vapor which is generated by evaporation. The remaining liquid distilland reaching the top of the evaporator is collected in an outlet plenum and either recycled to the inlet plenum or passed on to a second stage evaporator. The vapor separates from the liquid in the outlet plenum and, thereafter, is either condensed or used as a heating medium in a subsequent stage or effect.
The heat transfer coefficient achieved with the evaporators of the type just described are typically on the order of 1000 BTU/(hr)(ft.sup.2)(.degree.F.). The heat transfer coefficient may be increased, however, by adding a foam-producing surfactant to the distilland and reducing the pressure on the distilland as it enters the inlet end of the heat transfer tubes. The resulting foam enhances the heat transfer rate to the liquid by from 100-200%. The foamy annular layer flows over the inner wall of the tube while a vapor phase core separates along the axis of the tube and is able to flow to the outlet unhindered by the liquid phase flow. Such flow dynamics reduce the pressure drop through the tube bundle, allowing operating at lower pressures which favor evaporation. Moreover, the foamy nature of the liquid layer reduces its thermal resistance which further favors evaporation.
Hydrodynamic instability of the two-phase flow through the vertical tubes of the evaporator is problematic in both conventional and foamy flow. Such flow instability increases the pressure drop across the evaporator tubes which inhibits evaporation. Moreover, the hydrodynamic instability can lead to excessive mechanical vibration which can damage the evaporator as well as attached piping.
Hydrodynamic instability can result from a number of causes and is particularly troublesome during the start-up of a cold evaporator. Ideally, each tube in the evaporator tube bundle should operate with equal flow under approximately equal tube-side pressure drop. Instability results, however, when individual tubes or groups of tubes in the bundle experience surging or pulsating flow which may result from a variety of disturbances. Most commonly, excessive liquid will build up in a tube, and flashing of the liquid to vapor causes liquid to flow downward. The liquid which is expelled into the inlet plenum increases the hydrostatic pressure in that plenum which in turn causes a surge of liquid into the non-flashing tubes. Excess liquid then accumulates in the non-flashing tubes which can cause further flashing, eventually leading to an oscillating cross flow between the tubes at the tube inlet ends. This problem is most acute during start-up of the evaporator when the tubes are filled only with liquid and may heat at different rates. Flashing almost always initiates in individual tubes or groups of tubes prior to initiating in others, rendering evaporator start-up difficult or impossible. The problem is particularly acute in large evaporators (i.e., those having more than about 20 tubes) since differential heating of the tubes is virtually unavoidable.
Vertical tube foaming evaporators are somewhat less prone to the flow disturbances just described than are non-foamy flow evaporators, but such instability remains a serious concern. Such evaporators typically employ a distributor plate spaced below the inlets to the heat transfer tubes. The distributor plate includes a number of orifices which induce foaming as the distilland passes through. The spacing of the distributor plate, however, creates a manifold which can transmit flow disturbances from one tube or group of tubes to another, exacerbating the problems just described.
It is therefore desirable to provide a method and a system for operating vertical tube evaporators, and in particular vertical tube foaming evaporators, which effectively inhibits such undesirable flow characteristics from arising and which can dampen such flow once it has begun.
2. Description of the Prior Art
U.S. Pat. No. 3,846,254 to Sephton describes the construction and use of both upflow and downflow vertical tube evaporators as well as the practice of vertical tube foaming evaporation. The distance or gap between the distributor plate and the inlet ends of the tubes is taught to be up to several inches to provide for maximum turbulence and foaming immediately upstream of the tube inlets. Sephton, Desalination (1975) 16:1-13, discloses an upflow vertical tube evaporator having a distributor plate with a three inch gap between the distributor plate and the tube inlets. Sephton, Proc. of the Fourth Int. Symposium on Fresh Water from the Sea (1973) 1:471-480, discusses the problem of hydrodynamic instability in an upflow vertical tube foaming evaporator having an 18-tube bundle. U.S. Department of Commerce Publication (NTIS) PB-271 022, "Renovation of Power Plant Cooling Tower Blowdown for Recycle by Evaporation Crystallization with Interface Enhancement," June 1977, discloses the use of an upflow vertical tube evaporator having a distribution plate with an experimentally variable gap between its upper surface and a lower tube sheet which defines the tube inlets. A conventional upflow evaporator employing a plurality of nozzles in a fixed plate located beneath the tube inlets is described in Swiss Pat. No. 188,608.